Severen Redwood

Add support for the XDG Base Directory Specification

2

You can find the specification [here](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html). This [blog post](https://ploum.net/207-modify-your-application-to-use-xdg-folders/) lists the advantages of using the XDG Base Directory Specification as follows: - `$HOME` is a lot less cluttered. - Backups...

Correct the value of FAN_MARK_IGNORE

2

This PR aligns the value of `FAN_MARK_IGNORE` in libc with that in [`uapi/linux/fanotify.h`](https://github.com/torvalds/linux/blob/741e9d66/include/uapi/linux/fanotify.h#L87). Previously, it was incorrectly set to the value of `FAN_MARK_MOUNT`, which can cause subtle but catastrophic bugs...

There is currently no way to store or change settings. A TOML configuration file stored in the user's `XDG_CONFIG` directory would be the preferred way to implement this.

Sharing the ELPA cache would be a nice feature to have. Once this is implemented it will be an optional feature which can be turned off. (see #2)